This invention relates to hot rolling mills and particularly to those used continuous casting of thin steel strip a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting steel strip in a twin roll caster, the strip leaves the nip at very high temperatures on the order of 1400° C. or higher. If exposed to normal atmosphere, it would suffer very rapid scaling due to oxidation at such high temperatures. Therefore, a sealed enclosure is provided beneath the casting rolls to receive the hot strip and through which the strip passes away from the strip caster, the enclosure containing an atmosphere which inhibits oxidation of the strip. The oxidation inhibiting atmosphere may be created by injecting a non-oxidizing gas, for example, an inert gas such as argon or nitrogen, or combustion exhaust gases which may be reducing gases. Alternatively, the enclosure may be sealed against ingress of oxygen containing atmosphere during operation of the strip caster. The oxygen content of the atmosphere within the enclosure is then reduced during an initial phase of casting by allowing oxidation of the strip to extract oxygen from the sealed enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855. A thin strip is generally hot rolled in a hot rolling mill after the strip emerges from the caster to shape the thin strip.
It was generally understood in the past that to produce rolled product with acceptable shape in the hot rolling mill in the caster, as well as in other applications of the hot rolling mill, the mill roll gap profile should as near as possible to the cross-sectional profile of the incoming strip. Significant deviations between the incoming product profile and roll gap profile produces a localized shape defect in the outgoing rolled strip at the location across the strip width where the deviations occurs. If the roll gap thickness is less than the incoming strip thickness in that region relative to adjacent regions across the strip width, a buckle or loose defect occurs in the strip shape in that area producing a shape defect. This shape defect is characterized by lower than average tension stress. If the roll gap thickness is greater than the incoming strip thickness in that region relative to adjacent regions across the strip width, a tight or ridge strip shape defect will be produced in that area of the strip. The tight or ridge shape is characterized by higher than average tension stress.
The roll gap profile is determined principally by the ground profile of the work rolls. However, under operating load, the roll gap profile is also determined by the material characteristics of the incoming strip, the ground profiles of the mill rolls, the stack deflection of the mill rolls, mill housing stretch, and thermal effects due to heat generated in the roll gap by rolling. In the past, mill actuators have been provided that are capable of dynamically influencing the roll gap profile by some compensation for these parameters to make the roll gap profile better approach conformance to the incoming strip profile. For example, work roll bending jacks have been provided to affect symmetrical changes in the roll gap profile central region of the work rolls relative to regions adjacent the edges. The roll bending is capable of correcting symmetrical shape defects that are common to the central region and both edges of the strip. Also, force cylinders can affect asymmetrical changes in the roll gap profile on one side relative to the other side. The roll force cylinders are capable of skewing or tilting the roll gap profile to correct for shape defects in the strip that occur asymmetrically at either side of the strip, with one side being tighter and the other side being looser than average tension stress across the strip. Another device available is the use of spray nozzles at the edges of the strip, particularly thicker strip, to adjust for crown drop at the edges of the strip by cooling in such way that the edges drop caused by the transverse flow behavior of the material during rolling can be reduced or minimized. See U.S. Pat. No. 5,799,523.
In the course of hot mill rolling, in a cast strip plant and elsewhere, local shape defects can still occur in the strip that are not correctable by roll bending, roll force cylinder skewing or edge spraying of the strip. Typical examples of such shape defects are quarter buckle, local pocket buckles, local tight ridges or edge wave defects.
The present invention provides a method and apparatus for localized modification of the roll gap profile to correct these types of shape defects By controlling the localized cooling of the work surface of the work roll in zones across the work roll, both the upper and lower work roll profiles can be controlled by thermal expansion or contraction of the work rolls sufficient to produce thin steel strip without pronounced shape defects and without causing localized buckling, ridges and edge wave in the strip. We have found that it is possible to change the outlying areas of the strip profile in a zone or zones across the work roll, both high areas and low areas, relative to adjacent regions, and that that can be done without causing localized buckling, ridges and edge wave in the thin strip. This is done by changing the profile of the work surface of the work roll by localized control of the cooling of a zone or zones of the work roll where the shape defect is observed relative to adjacent regions
Specifically, the control of localized cooling can be accomplished by increasing the relative volume or velocity, or decreasing of temperature, of coolant sprayed through nozzles onto the work roll surfaces in the zone or zones of an observed strip shape buckle or loose area, causing the work roll diameter of either or both of the work rolls in that area to contract, increasing the roll gap profile, and effectively tightening the strip shape in that region to alleviate pronounced defects in the strip without causing localized buckling, ridges and edge wave in the strip. Conversely, by decreasing the relative volume or velocity, or increasing the temperature, of the coolant sprayed by the nozzles onto the work surfaces of the work rolls in the zone or zones where shape ridge or tight area is observed in the strip, the work roll diameter in that area expands, decreasing the roll gap profile, and effectively loosening the strip shape in that area to alleviate pronounced defects in the strip without causing localized buckling, ridges and edge wave in the strip. Alternatively or in combination, the control of localized cooling can be accomplished by internally controlling cooling the work surface of the work roll in zones across the work roll by localized control of temperature or volume water circulated through the work rolls adjacent the work surfaces.
A method is provided for controlling strip shape in a hot rolling mill comprising:                a. assembling a hot mill having work rolls with work surfaces forming a gap between them through which hot strip is rolled, said work rolls having roll work surfaces relating to a desired strip profile to be rolled;        b. localized controlling of cooling of the work surface of the work rolls in at least three lateral zones, a central and two edge zones;        c. rolling strip between the work rolls; and        d. varying the localized cooling in each zone along the work rolls so as to locally control the shape and inhibit the formation of local shape defects in the strip rolled by the hot rolling mill.        
Moreover, five zones, a central zone, two edge zones and two intermediate or quarter zones, can be provided, so that quarter buckles can be inhibited which is a particularly difficult problem in hot rolling. The number of zones provided may be increased to the number of localized cooling devices that the geometry adjacent the work rolls permit to be positioned in a particular embodiment. There can be more than one row of localized cooling devices, e.g., two or more rows of nozzles, positioned adjacent the work surface so that adjacent zones may not be necessarily serviced by localized cooling devices in the same row. The localized cooling devices can be positioned to overlap so that the regions of the work roll surface are covered at least across the portion where strip engages the work rolls, and typically beyond where strip engages the work rolls, to provide for effective localized controlling of cooling of the work roll surface being controlled. Conversely, a zone could be serviced by two or more localized cooling devices not all of which are necessarily capable of individual control. There needs to be, however, localized controlling of cooling of the work surface of the work rolls in each zone to provide effective control of the strip shape. In other words, the zones of controlled localized cooling should be such as to cover the surface of the work roll at least in the area where the strip encounters the work roll, but may extend substantially beyond the edge of the strip, and may include the entire work roll, to provide effective control of the strip shape.
On the other hand, if sprays from nozzles are used as the localized cooling devices, the sprays from nozzles onto the work surface should not impinge on each other because in so going the sprays can interfere with each other and reduce effective control in shaping of the work roll within a zone, and in turn reduce effective control the shape defects observed in the strip of the strip Whether the control of localized cooling is accomplished by one or more sprays or internal circulation of coolant within each zone, the coolant is typically water, although other coolants may be utilized as desired. The localized controlling of cooling of the work surface of the work rolls in each zone thus can be accomplished by varying the volume, velocity and temperature of coolant impacting on the work surface of the work rolls in each zone.
The method of controlling strip shape in a hot rolling mill may comprise the additional steps of:                e. sensing downstream of the hot rolling mill the shape in the strip in each zone; and        f. automatically varying the cooling by the localized cooling device(s) in each zone through a control system controlled by the strip shape sensed in the strip in each zone downstream of the hot rolling mill.        
The strip shape in each zone may be measured by a tension roll, laser measuring distance or any other suitable device to dynamically measure strip shape.
Additionally or alternatively, a method of producing thin cast strip with a controlled strip shape by continuous casting is provided comprising the steps of:                a. assembling a thin strip caster having a pair of casting rolls having a nip there between;        b. assembling a metal delivery system capable of forming a casting pool between the cast rolls above the nip with side dams adjacent the ends of the nip to confine said casting pool;        c. assembling adjacent the thin strip caster a hot rolling mill having work rolls with work surfaces forming a gap between them through which hot strip from the caster is rolled, said work rolls having work roll surfaces relating to a desired strip shape to be roll;        d. assembling localized cooling devices positioned at intervals along the work surfaces of the work rolls of the hot rolling mill in at least three lateral zones, a central and two edge zones, and capable of localized cooling the work surface of at least one of the work rolls;        e. assembling a control system capable of individually regulating the cooling by the localized cooling devices within zones on the work surface of at least one work roll of the hot rolling mill;        f. introducing molten steel between the pair of casting rolls to form a casting pool supported on casting surfaces of the casting rolls confined by side dams;        g. counter-rotating the casting rolls to form solidified metal shells on the surfaces of the casting rolls and cast thin steel strip through the nip between the casting rolls from said solidified shells; and        h rolling the thin cast strip between the work rolls of the hot rolling mill and varying the cooling within each zone so as to control the shape of the work rolls and inhibit the formation of shape defects encountered in the thin cast strip in the zones.        
Again, importantly, five zones, a central zone, two edge zones and two intermediate or quarter zones, can be provided so that quarter buckles can be inhibited. As with the hot rolling mill in other embodiments, the number of zones provided may be increased to the number of localized cooling devices that the geometry adjacent the work rolls permit to be positioned in a particular embodiment. There can be more than one row of localized cooling devices, e.g., two or more rows of nozzles, positioned adjacent the work surface so that adjacent zones may not necessarily be serviced by localized cooling devices, e.g., nozzles, in the same row. The localized cooling devices can be positioned to overlap so that the regions of the work roll surface are covered at least across the portion where strip engages the work rolls, and typically beyond where strip engages the work rolls, to provide for effective localized controlling of cooling of the work surface control. Conversely, a zone could be serviced by two or more localized cooling devices not all of which are necessarily capable of individual control. There need to be, however, localized controlling of cooling of the work surface of the work rolls in each zone to provide effective control of the strip shape. In other words, the zones of controlled localized cooling should be such as to cover the surface of the work roll at least in the area where the strip encounters the work roll, but may extend substantially beyond the edge of the strip, and may include the entire work roll, to provide effective control of the strip shape.
On the other hand, if sprays from nozzles are used as the localized cooling devices, the sprays from nozzles on the work surface should not impinge on each other, because in so going the sprays can interfere with each other and reduce effective control in shaping of the work roll within a zone, and in turn reduce effective control the shaping of defects observed in the strip of the strip. Again, whether the control of localized cooling is accomplished by one or more sprays or internal circulation of coolant within each zone, the coolant is typically water, although other coolants may be utilized as desired. The localized controlling of cooling of the work surface of the work rolls in each zone thus can be accomplished by varying the flow volume, velocity and temperature of coolant impacting on the work surface of the work rolls in each zone.
There, the method of producing thin cast strip with a controlled strip shape by continuous casting may comprise the additional steps of:                i. sensing downstream of the hot rolling mill the shape on the strip in each zone; and        j. automatically varying the cooling by at least one of the localized cooling devices in each zone through a control system controlled by the strip shape sensed in the strip in each zone downstream of the hot rolling mill.        
A hot rolling mill is also provided comprising:                a. work rolls with work surfaces forming a gap between them through which hot strip is rolled, said work rolls having work roll surfaces relating to a desired strip profile to be rolled;        b. cooling devices positioned at intervals across the work rolls in at least three lateral zones, a central and two edge zones, and capable of individually regulating the cooling of the work rolls in each zone; and        c. a control system capable of individually regulating the cooling devices within each zone so as to control the shape profile of the work surfaces of the work rolls in each zone, and in turn inhibit the formation of shape defects in strip rolled by the hot rolling mill in each zone.        
In the hot rolling mill, there may be in particular five zones, a central zone, two edge zones and two intermediate or quarter zones, along the working surface of the work roll to control for quarter buckles in the strip which is a particular problem. As with the method of operating the hot rolling mill, the number of zones that can be provided in the hot rolling mill can be expanded to the number of localized cooling devices that can be provide with the geometry of a particular embodiment. There can be two or more rows of localized cooling devices positioned adjacent the work surface so that devices in a row may not necessarily serviced adjacent zones. The localized cooling devices can be thus easily positioned to overlap so that the regions of the work roll surface are covered at least across the portion where strip engages the work rolls, and typically beyond where strip engages the work rolls, to provide for effective localized controlling of cooling of the work surface. Conversely, a zone could be serviced by two or more localized cooling devices not all of which are capable of individual control. In any case, the localized cooling in each zone may overlap between zones at the work surface of the work rolls to provide effective control of the shape of the work roll surface and the strip shape. In another words, the zones of controlled localized cooling should be such as to cover the surface of the work roll at least in the area where the strip encounters the work roll, but may extend substantially beyond the edge of the strip, and may include the entire work roll, to provide effective control of the strip shape. On the other hand, if sprays from nozzles are used as the localized cooling devices, the sprays from nozzles onto the work roll surface should not impinge on each other to provide effective control in shaping of the work roll surface within a zone, and in turn control the shape defects observed in the strip in each zone Whether the control of localized cooling is accomplished by sprays or circulation in the hot rolling mill, the coolant is typically water, with other coolants utilized as desired. The localized controlling of cooling of the work surface of the work rolls in each zone thus can be accomplished by varying the flow volume, velocity and temperature of coolant impacting on the work surface of the work rolls in each zone.
A thin cast strip plant may also be provided for producing strip with a controlled strip shape by continuous casting comprising:                a. a thin strip caster having a pair of casting rolls having a nip there between;        b. a metal delivery system capable of forming a casting pool between the cast rolls above the nip with side dams adjacent the ends of the nip to confine said casting pool;        c. a hot rolling mill adjacent the thin strip caster having work rolls with work surfaces forming a gap between them through which hot strip can be rolled, said work rolls having work roll surfaces relating to a desired strip profile to be rolled;        d. a plurality of localized cooling devices positioned at intervals along the work rolls of the hot rolling mill capable of localized control of cooling of the work surface of at least one of the work rolls in at least three lateral zones, a central and two edge zones, with the cooling of the work surface within each zone being capable of being individually controlled;        e. a drive capable of counter-rotating the casting rolls to form solidified metal shells on the surfaces of the casting rolls and cast thin steel strip through the nip between the casting rolls from said solidified shells; and        f. a control system capable of individually regulating localized control of cooling of the work surface by cooling device(s) in each zone of the work roll so as to control the shape of the work surface and inhibit the formation of shape defects in the thin cast strip in each zone.        
There may be in particular five zones, a central zone, two edge zones and two intermediate or quarter zones, to control for quarter buckles in the strip. Again, the number of zones that can be provided in the hot rolling mill can be expanded to the number of localized cooling devices that can be provide with the geometry of a particular embodiment. There can be more than one row of localized cooling devices, e.g., two or more rows of nozzles, positioned adjacent the work surface so that adjacent zones may not necessarily be serviced by localized cooling devices in the same row. The localized cooling devices can be thus easily positioned to overlap so that the regions of the work roll surface are covered at least across the portion where strip engages the work rolls, and typically beyond where strip engages the work rolls, to provide for effective localized controlling of cooling of the work surface control. Conversely, a zone could be serviced by two or more localized cooling devices not all of which are capable of individual control. In any case, the localized cooling by the localized cooling devices in each zone may overlap between zones at the work surface of the work rolls to provide effective control of the shape of the work roll surface and the strip shape, In another words, the zones of controlled localized cooling should be such as to cover the surface of the work roll at least in the area where the strip encounters the work roll, but may extend substantially beyond the edge of the strip, and may include the entire work roll, to provide effective control of the strip shape. On the other hand, if sprays from nozzles are used as the localized cooling devices, the sprays from nozzles onto the work roll surface should not impinge on each other to provide effective control in shaping of the work roll surface within a zone, and in turn effective control the shaping of defects observed in the strip in each zone Whether the control of localized cooling is accomplished by sprays or internal coolant circulation in the work rolls of the hot rolling mill, the coolant is typically water, with other coolants utilized as desired. The localized controlling of cooling of the work surface of the work rolls in each zone thus can be accomplished by varying the volume, velocity and temperature of coolant impacting on the work surface of the work rolls in each zone.
In each embodiment of the method, mill and plant, the localized controlling of cooling of the work surface of the work roll needs to have the thermal effect to locally expand and contract the diameter the work roll to make a substantial change in the roll gap, and affect the desired local roll shape control in each zone. This localized controlling of cooling of the work surface of the work roll is in addition to the coolant that may be sprayed on the work roll at the same time to cool the work roll. In any case, the effectiveness on the strip shape control depends upon the temperature differential between the work roll surface and the coolant, as well as the coolant volume and velocity sprayed onto or circulated within the particular zone of the work surface of the work roll. The coolant practice may be modified to minimize the fixed coolant volume while maintaining work roll surface temperatures within an acceptable range, e.g. below 120° C. (250° F.), while the strip is typically about 1200° C. The devices controlling localized cooling used for shape control may be turned completely off at the start of a casting campaign, or set at some intermediate level so that regulating the localized cooling devices can both expand and contract the work roll diameter in a zone. As local areas with loose shape are observed, the localized cooling devices can be regulated adjacent the upper and lower work rolls in the zone corresponding to where a shape defect is observed to contract or expand the work roll diameter, increase or decrease the relative roll gap and tighten or loosen the strip shape coming out of the hot rolling mill.
If spray nozzles used as the localized cooling devices, with controllable nozzles at about every two inches across the strip width, the ratio of coolant volume for shape defect correction to uncontrolled spray volume for cooling of the work rolls is in the range from about 1 to 1 up to 3 to 1. In another words, the volume of the coolant from the controllable nozzles to effect localized cooling in each zone may be about 100% to 300% of the volume from the nozzles that are constantly spraying the work surface to cool the work roll. The actual total gallon per minute flow depends on the thickness of the strip being made, upon whether both the upper and lower work are being locally cooled, upon the setting of the main upper and/or lower work roll coolant supply valves, the temperature of the coolant, and the sizes of the individual spray nozzles (which may be the same or different for the spray nozzles used to generally cool the work rolls).
The hot rolling mill may automatically control strip shape by providing sensors in a sensor roll positioned downstream of the mill to sense localized shape in the strip along the width of the strip, and a control system capable of controlling the flow of coolant sprayed on the work rolls in the individual zones through controllable localized cooling devices. By this arrangement, the hot rolling mill can automatically adjust for shape defects sensed in the strip by regulating the localized cooling in each zone in the individual zones along the work surface of the work roll, and in turn controlling the shape of the surface of the work rolls of the mill and the shape of the thin steel strip.